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Chapter 1 Magnetic Circuits and Magnetic Materials

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Chapter 1 Magnetic Circuits and Magnetic Materials Chapter 1 Magnetic Circuits and Magnetic Materials The objective of this course is to study the devices used in the interconversion of electric and mechanical energy, with emphasis placed on electromagnetic rotating machinery. The transformer, although not an electromechanical-energy-conversion device, is an important component of the overall energy-conversion process. Practically all transformers and electric machinery use ferro-magnetic material for shaping and directing the magnetic fields that acts as the medium for transferring and converting energy. Permanent-magnet materials are also widely used. The ability to analyze and describe systems containing magnetic materials is essential for designing and understanding electromechanical-energy-conversion devices. The techniques of magnetic-circuit analysis, which represent algebraic approximations to exact field-theory solutions, are widely used in the study of electromechanical-energy-conversion devices. §1.1 Introduction to Magnetic Circuits Assume the frequencies and sizes involved are such that the displacement-current term in Maxwell’s equations, which accounts for magnetic fields being produced in space by time-varying electric fields and is associated with electromagnetic radiations, can be neglected. Z H : magnetic field intensity, amperes/m, A/m, A-turn/m, A-t/m Z B : magnetic flux density, webers/m2, Wb/m2, tesla (T) Z 1 Wb =108 lines (maxwells); 1 T =104 gauss Z From (1.1), we see that the source of H is the current density J . The line integral of the tangential component of the magnetic field intensity H around a closed contour C is equal to the total current passing through any surface S linking that contour. O Hdl = O J ⋅ da (1.1) c s Z Equation (1.2) states that the magnetic flux density B is conserved. No net flux enters or leaves a closed surface. There exists no monopole charge sources of magnetic fields. O B ⋅ da = 0 (1.2) s A magnetic circuit consists of a structure composed for the most part of high-permeability magnetic material. The presence of high-permeability material tends to cause magnetic flux to be confined to the paths defined by the structure. Figure 1.1 Simple magnetic circuit. 1 Z In Fig. 1.1, the source of the magnetic field in the core is the ampere-turn product N i , the magnetomotive force (mmf) F acting on the magnetic circuit. Z The magnetic flux φ (in weber, Wb) crossing a surface S is the surface integral of the normal component B : φ = O B ⋅ da (1.3) s Z φ c : flux in core, Bc : flux density in core φ = c Bc Ac (1.4) Z H c : average magnitude H in the core. The direction of H c can be found from the RHR. F = Ni = O Hdl (1.5) = = F Ni H clc (1.6) The relationship between the magnetic field intensity H and the magnetic flux density B : B = μH (1.7) Z Linear relationship? Z μ = μ μ μ r 0 , : magnetic permeability, Wb/A-t-m = H/m Z μ = π × −7 0 4 10 : the permeability of free space Z μ r : relative permeability, typical values: 2000-80,000 A magnetic circuit with an air gap is shown in Fig. 1.2. Air gaps are present for moving elements. The air gap length is sufficiently small. φ : the flux in the magnetic circuit. Figure 1.2 Magnetic circuit with air gap. φ = Bc (1.8) Ac φ = Bg (1.9) Ag = + F H clc H g lg (1.10) B B F = c l + g g (1.11) μ c μ 0 C l g S F = φ D c + T (1.12) D μ μ T E Ac 0 Ag U 2 Z Rc , Rg : the reluctance of the core and the air gap, respectively, l g R = c , R = (1.13), (1.14) c μ g μ Ac 0 Ag = φ( + ) F Rc Rg (1.15) F φ = (1.16) + Rc Rg F φ = (1.17) l c + g μ μ Ac 0 Ag Z In general, for any magnetic circuit of total reluctance Rtot , the flux can be found as F φ = (1.18) Rtot Z The permeance P is the inverse of the reluctance = 1 Ptot (1.19) Rtot Z Fig. 1.3: Analogy between electric and magnetic circuits: Figure 1.3 Analogy between electric and magnetic circuits: (a) electric ckt, (b) magnetic ckt. Z << << Note that with high material permeability: Rc Rg and thus Rtot Rg , F Fμ A μ A φ ≈ = 0 g = Ni 0 g (1.20) Rg g g Z Fig. 1.4: Fringing effect, effective Ag increased. Figure 1.4 Air-gap fringing fields. 3 In general, magnetic circuits can consist of multiple elements in series and parallel. = = = F O Hdl B Fk B H k lk (1.21) k k F = O J ⋅ da (1.22) s = V B Rk ik (1.23) k = Bin 0 (1.24) n φ = B n 0 (1.25) n Figure 1.5 Simple synchronous machine. 4 §1.2 Flux Linkage, Inductance, and Energy Faraday’s Law: d O E ⋅ ds = − O B ⋅ da (1.26) c dt s Z λ : the flux linkage of the winding, ϕ : the instantaneous value of a time-varying flux, Z e : the induced voltage at the winding terminals dϕ dλ e = N = (1.27) dt dt λ = Nϕ (1.28) L : the inductance (with material of constant permeability), H = Wb-t/A λ L = (1.29) i N 2 L = (1.30) Rtot Z The inductance of the winding in Fig. 1.2: N 2 N 2 μ A L = = 0 g (1.31) ()μ g / 0 Ag g Figure 1.6 (a) Magnetic circuit and (b) equivalent circuit for Example 1.3. 5 6 Magnetic circuit with more than one windings, Fig. 1.8: Figure 1.8 Magnetic circuit with two windings. = + F N1i1 N 2i2 (1.32) μ A φ = ()N i + N i 0 c (1.33) 1 1 2 2 g C μ A S C μ A S λ = N φ = N 2 D 0 c T i + N N D 0 c T i (1.34) 1 1 1 E g U 1 1 2 E g U 2 λ = + 1 L11i1 L12i2 (1.35) μ A L = N 2 0 c (1.36) 11 1 g μ A L = N N 0 c = L (1.37) 12 1 2 g 21 C μ A S C μ A S λ = N φ = N N D 0 c T i + N 2 D 0 c T i (1.38) 2 2 1 2 E g U 1 2 E g U 2 λ = + 2 L21i1 L22i2 (1.39) μ A L = N 2 0 c (1.40) 22 2 g Z Induced voltage, power (W = J/s), and stored energy: d e = ()Li (1.41) dt d e = L ()Li (1.42) dt di dL e = L + i (1.43) dt dt dλ p = ie = i (1.44) dt t λ ΔW = 2 p dt = 2 i dλ (1.45) O Oλ t1 1 λ λ 2 2 λ 1 ΔW = i dλ = dλ = ()λ2 − λ2 (1.46) Oλ Oλ 2 1 1 1 L 2L 1 L W = λ2 = i 2 (1.47) 2L 2 7 §1.3 Properties of Magnetic Materials The importance of magnetic materials is twofold: Z Magnetic materials are used to obtain large magnetic flux densities with relatively low levels of magnetizing force. Z Magnetic materials can be used to constrain and direct magnetic fields in well-defined paths. Ferromagnetic materials, typically composed of iron and alloys of iron with cobalt, tungsten, nickel, aluminum, and other metals, are by far the most common magnetic materials. Z They are found to be composed of a large number of domains. Z When unmagnetized, the domain magnetic moments are randomly oriented. Z When an external magnetizing force is applied, the domain magnetic moments tend to align with the applied magnetic field until all the magnetic moments are aligned with the applied field, and the material is said to be fully saturated. Z When the applied field is reduced to zero, the magnetic dipole moments will no longer be totally random in their orientation and will retain a net magnetization component along the applied field direction. The relationship between B and H for a ferromagnetic material is both nonlinear and multivalued. Z In general, the characteristics of the material cannot be described analytically but are commonly presented in graphical form. Z The most common used curve is the B − H curve. Z Dc or normal magnetization curve: Z Hysteresis loop (Note the remanance): 8 Figure 1.9 B-H loops for M-5 grain-oriented electrical steel 0.012 in thick. Only the top halves of the loops are shown here. (Armco Inc.) Figure 1.10 Dc magnetization curve for M-5 grain-oriented electrical steel 0.012 in thick. (Armco Inc.) Figure 1.13 Hysteresis loop. 9 §1.4 AC Excitation In ac power systems, the waveforms of voltage and flux closely approximate sinusoidal functions of time. We are to study the excitation characteristics and losses associated with magnetic materials under steady-state ac operating conditions. Z Assume a sinusoidal variation of the core flux ϕ(t) : ϕ()= φ ω = ω t max sin t Ac Bmax sin t (1.48) φ = ϕ where max amplitude of core flux in webers = Bmax amplitude of flux density Bc in teslas ω = angular frequency = 2πf f = frequency in Hz Z The voltage induced in the N-turn winding is ()= ω φ (ω )= ω e t N max cos t Emax cos t (1.49) E = ωNφ = 2πfNA B max max c max (1.50) Z The Root-Mean-Squared (rms) value: C 1 T S F = D O f 2 ()t dt T (1.51) rms E T o U π = 2 = π Erms fNAc Bmax 2 fNAc Bmax (1.52) 2 Note that the rums value of a sinusoidal wave is 12 times its peak value.
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